Although nonlinear energy harvesting systems have been the subject of extensive research, a clear and efficient parametric design methodology remains underdeveloped. Conventional approaches, such as harmonic balance and numerical simulations, often fail to establish explicit relationships between energy harvesting performance and system parameters, and they typically demand substantial computational resources for parametric analysis. This study introduces a novel frequency-domain framework based on the Volterra series for analyzing a representative nonlinear energy harvesting system. A concise parametric characterization technique, termed the Linear and Nonlinear Characteristic Output Spectrum (lnCOS) method, is proposed to describe both the nonlinear output frequency response and the power generation function (PGF) of the system. The lnCOS function and the associated Parametric-PGF formulation offer a direct and computationally efficient means of predicting vibration responses and power output without the need to solve complex algebraic systems. By explicitly linking power generation to key structural parameters, this method provides a transparent and analytically tractable design tool. To the best of the authors’ knowledge, this is the first study to derive closed-form expressions for both vibration response and power generation in nonlinear energy harvesting systems based on structural parameters—an achievement not attainable through traditional methods.